1. Field of the Invention
The present invention relates to the filtering of animated digital images which are obtained after coding and decoding by transformation of pixel blocks respectively in a transmitting terminal and a receiving terminal, the purpose of these coding and decoding operations being to reduce the data bit rate in a transmission line linking the two terminals.
2. Description of the Prior Art
Current studies in telecommunications networks are notably oriented towards the transmission of images between subscribers. To do this, in view of the infrastructure of existing networks, it is necessary to use transmission means with a limited frequency bandwidth. For indicative purposes, an animated television image requires a digital bit rate of the order of a hundredth of Mbit/s, whereas basic access to the Integrated Services Digital Network (ISDN) offers two channels at a rate of 64 kbit/s, one being intended for the transmission of sound and the other, in such applications, for the transmission of images. For such a low transmission bit rate that can be typically included between 64 and 1,920 kbit/s, coding and decoding devices have been provided in the transmitter and receiver in order to compress the image to be transmitted, and more precisely to limit the data to be transmitted in the transmission line while correctly reconstituting the animated image by the receiver. Three main techniques have been advocated to meet these requirements:
a) coding by prediction: this consists in the transmission of a prediction error by the transmitter to the receiver, to be added to the predicted value of the image point derived in the receiver, the prediction algorithms being identical in the transmitter and receiver;
b) coding by transformation: this consists in representing the image in a different space of the image plane, for easier separation of redundant and relevant data of the image and for transmission of the latter only; and
c) coding by approximation: this consists in deducing, from a part of the transmitted image, the other part of the image.
These three methods are combinable and separately call on three distinct operations, namely transformation, quantization and coding.
International standardization organizations, such as the CIE, CCITT and ISO, decided in 1988 to draw up a standard for the coding of fixed and animated images based on prediction, compensation of motion and discrete cosine transformation DCT (close to the Fourier transform). DCT transformation offers data compressing qualities, simplicity of use and compatibility with the standards being developed in the field of animated images (Visiophone (registered trade mark), digital television, . . . ).
Discrete cosine transformation consists in passing from a "visual" marking in which each picture element, called pixel, represents a brightness level varying e.g. from 0 to 255, to a "transformed" marking of the same dimension. The complexity in performing such operators being a function of the dimension (I.N)(J.N) of the image to be processed, the transformation applies to a plurality (I.J) of square blocks of pixels with dimension N.sup.2 in the digitalized image, N, I and J being integers, and (I.N) and (J.N) denoting the number of lines of pixels and the number of columns of pixels in an image. Furthermore, as the function of the discrete cosine transform is to operate in a two-dimensional space where the coefficients are uncorrelated, a maximum level of uncorrelation between pixels of the image plane and "pixels" of the transformed plane is obtained thereby determining image blocks in which the pixels have a high level of correlation, which intuitively corresponds effectively to small image areas having substantially the same characteristics as regards brightness, color, etc. Typically, the values of N are equal to 8, 16 or 32.
This dividing into blocks of the digitalized image leads, at the level of the boundaries separating these blocks, to disturbance effects resulting in a mosaic impression in the image reconstituted by the receiving terminal.
These disturbance effects produce an appearance of block structure of the image, and are generated by:
discontinuities in brightness, or edge effects, at the boundaries of the blocks. This phenomenon is due to the fact that each block is virtually translated in the course of the transformation into an infinite and periodic (even) sequence for obtaining a periodic spectrum, and therefore transform coefficients in finite number; and
an inter-block noise generated by quantization errors from a given block to an adjacent block, quantizations being applied on the transform coefficients.
For purely explanatory purposes, FIGS. 1A and 1B show two square blocks, BL1 and BL2, which are consecutive in an image and each have N.sup.2 =64 pixels, transmitted by a transmitter and reconstituted by a receiver. The image is presupposed to be in black and white. A closed curve CF in a dark line shows the appearance, as perceived by the eye, of the contour of a set of black pixels in the two blocks. A magnifying glass LO shows, by comparison with FIG. 1A, that in FIG. 1B, a break in the continuity of the curve is induced by these disturbance effects. The effect produced at the level of the entire image will be intuitively conceived.